Integrated circuit capacitor

ABSTRACT

An integrated circuit with a capacitor includes a conductive substrate, a layer of field dielectric formed on the conductive substrate, a layer of conductive metal or conductive polycrystalline silicon formed on the field dielectric, and first and second laterally spaced apart layers of conductive material formed on the conductive metal or polycrystalline silicon. Each spaced apart layer preferably includes a layer of titanium nitride disposed over a layer of titanium. A layer of capacitor dielectric is deposited on the first of the spaced apart layers, and metal is deposited over the capacitor dielectric and the second layer of conductive material.

This is a divisional of application Ser. No. 08/353,015 filed Dec. 9,1994, now U.S. Pat. No. 5,508,415.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for fabricating a capacitor,and more particularly, to a method for fabricating a metal to metalcapacitor in an integrated circuit.

2. Background of the Art

Capacitors are used extensively in electronic devices for storing anelectric charge. The capacitors essentially comprise two conductiveplates separated by an insulator. The capacitance, or amount of chargeheld by the capacitor per applied voltage, is measured in farads anddepends upon the area of the plates, the distance between them, and thedielectric value of the insulator. Capacitors are used in filters, inanalog-to-digital converters (ADCs), in memories, and various controlapplications.

Capacitors in integrated circuits are usually fabricated frompolysilicon, metal to polysilicon or metal to polycide structures. Inmost applications, such as in analogue to digital converters (ADC's),one needs capacitors whose capacitance does not vary with voltage. Ameasure of the variation of capacitance with applied voltage is calledthe voltage coefficient of capacitance (VOC) measured in parts permillion. Generally, VOC of capacitors used on integrated circuits is notzero (50-300 ppm) and hence needs to be nulled. Circuit techniques thatemploy null methods assume that the variation of VOC with voltage, whilenot zero, is a linear function of voltage, which is not a validassumption in integrated circuit capacitors. Furthermore, while thesetechniques increase precision and resolution of ADC's they consume chiparea, and hence increase chip cost. If the VOC of the capacitors is lessthan a few ppm one does not need to employ null circuit techniques,thereby reducing circuit complexity and cost.

U.S. Pat. No. 5,108,941 to Paterson et al. discloses a method of makinga metal to polysilicon type capacitor having a low VOC as compared topolysilicon type capacitors. In the Paterson et al. method the bottomplate of the capacitor is formed over a field oxide structure, and themultilevel dielectric is deposited thereover. The multilevel dielectricis removed from the capacitor area, and an oxide/nitride dielectric isdeposited over the exposed bottom plate and over the multilevel by wayof low pressure chemical vapor deposition ("LPCVD"). A first layer oftitanium/tungsten is preferably deposited prior to contact etch, and thecontacts to moat and unrelated polysilicon are formed. Metallization issputtered overall, and the metal and titanium/tungsten are cleared toleave the metallization filling the contact holes, and a capacitorhaving a titanium/tungsten and metal top plate.

In any fabrication process, simplicity is an advantage. Thus, afabrication method which can achieve the same or better quality productwith the same cost of materials while using fewer steps is highlypreferred, especially if elimination of fabrication steps reduces laborcosts and the need for expensive manufacturing equipment. A newstructure built from materials already being used in the fabricationprocess is preferred since it reduces materials development efforts andthe need for expensive manufacturing equipment.

Moreover it is desirable to have flexibility in the processing steps forfabricating integrated circuits. Particularly, it is highly advantageousto have a modular process for forming a capacitor, i.e. a process thatcan be added as an option to an existing digital process with no changesin sequence operations. Employing a silicided metal on polysilicon("polycide") as contemplated in U.S. Pat. No. 5,108,941 entailssiliciding the entire polysilicon layer to achieve the optimum voltagestability. However, siliciding sharply reduces processing flexibility.For example, with a silicided structure heat treatment of the integratedcircuit for such purposes as annealing, diffusion, driving in dopants,smoothing interlevel dielectrics and the like, is limited totemperatures below about 850° C. It would be advantageous to be able touse temperatures above 850° C. and to be able to form low VOC capacitorsat various stages of integrated circuit fabrication.

SUMMARY OF THE INVENTION

A low VOC metal to metal capacitor and a method for making same in anintegrated circuit are provided herein. The method includes forming afield dielectric layer on a conductive substrate; forming a bottom plateof the capacitor comprising a layer of first metal in electricalcontacting relation with said conductive polysilicon layer; forming alayer of capacitor dielectric in contact with said bottom plate of saidcapacitor; and forming a top plate of the capacitor comprising a layerof second metal onto said capacitor dielectric.

The bottom plate preferably comprises a layer of titanium with anoverlayer of titanium nitride. The top plate is preferably aluminum. Andthe capacitor dielectric is preferably silicon dioxide oroxide/dielectric

Preferably, the method further comprises forming a conductivepolycrystalline silicon layer in contact with said field dielectriclayer and depositing a pattern of interlevel dielectric (e.g., silicondioxide) on the conductive polycrystalline silicon layer prior toforming the bottom plate of the capacitor.

BRIEF DESCRIPTION OF THE DRAWINGS

So that one skilled in the art to which the subject invention appertainswill better understand how to practice the method of the presentinvention, preferred embodiments of the method will be described indetail hereinbelow with reference to the drawings wherein:

FIG. 1 is a cross sectional diagram of the capacitor 10 of the presentinvention.

FIGS. 2 to 6 are cross-sectional diagrams illustrating various steps inthe formation of the capacitor of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is a cross-sectional diagram of the metal to metal capacitor 10constructed according to the method described herein. The capacitor 10comprises a lower plate 14, dielectric layer 15, and upper plate 17.These layers are optionally stacked on top of a polycrystalline silicon("polysilicon") layer 12, which is formed on top of field oxide ("FOX")dielectric layer 11 disposed on a silicon substrate 18. The substrate 18may be a p-type or n-type silicon.

Lower plate 14 of the capacitor 10 can be formed from any metal suitablefor conducting and holding an electric charge, such as aluminum, copper,silver, titanium, or noble metals such as gold, platinum, palladium, andthe like. Preferably, lower plate 14 is a multilayered structurecomprising layer 14a of titanium (Ti) overcoated with layer 14b oftitanium nitride (TiN). Layer 14 may range in thickness from about 0.04microns to about 0.15 microns, with the Ti layer 14a ranging inthickness from about 0.01 microns to about 0.05 microns, and the TiNlayer ranging in thickness from about 0.03 microns to about 0.10microns.

Layer 15 can be formed from any suitable dielectric, such as silicondioxide (SiO₂) and/or silicon nitride and can generally range inthickness from about 0.01 microns to about 0.10 microns depending on theelectrical requirements of the capacitor.

The top plate 17 of the capacitor can be any metal suitable forfabricating capacitors on integrated circuits. Aluminum is a preferredmetal for fabricating top plate 17. The aluminum may optionally be dopedwith, for example, copper or silicon, or alternatively may be part of amultilayered metal system.

The method of making a metal to metal capacitor 10 having a very low VOCis illustrated in FIGS. 2 to 6.

Referring to FIG. 2, a plate of polysilicon. 12 is patterned over alayer 11 of field oxide (SiO₂). The field oxide layer 11 is formed onthe silicon substrate 18 in a manner well known in the art, such aslocal oxidation or by deposition. Likewise, patterning of polysilicon iswell known in the art. Then, interlevel dielectrics 13 are deposited inan appropriate pattern to insulate the polysilicon layer 12 fromoverlying metallization. The interlevel dielectric 13 is preferablysilicon dioxide, which may optionally be doped, or may optionally alsobe silicon nitride or any other material having properties suitable forthe use described herein. The structure at this point is as diagrammedin FIG. 2.

The polysilicon 12 is preferably heavily doped to be relativelyconductive, as in conventional integrated circuits. Generally thepolysilicon layer will be n-doped either by diffusion, ion implantation,or by in-situ doping. It should be noted that polysilicon layer 12 doesnot serve as the bottom plate of capacitor 10 and hence is optional.However, it serves as a lead to conduct electric charge to and from thebottom layer 14 and thereby facilitates the incorporation of capacitor10 into an integrated circuit. In contrast to the method disclosed inU.S. Pat. No. 5,108,941 the polysilicon layer 12 herein does not need tobe silicided to achieve optimum VOC performance of the capacitor 10.Moreover, other conductive materials may be substituted for polysilicon,such as aluminum, copper, silver, titanium, or noble metals such asgold, platinum, palladium, and the like.

Referring to FIG. 3, a bottom plate 14 is formed by depositing a layer14a of titanium (Ti) and 14b of titanium nitride (TiN), preferably bysputter deposition. The titanium nitride may be deposited in-situ aftera certain thickness of titanium metal has been deposited by bleedingnitrogen gas into the titanium sputtering chamber. Thus, the titaniumnitride forms a coating which covers the titanium and serves as an etchstop for later processing steps as discussed below.

Next, the capacitor dielectric 15 is deposited. The dielectric is formedby first depositing tetraethoxysilane ("TEOS") on the layer 14 of Ti/TiNby means of chemical vapor deposition ("CVD") or plasma enhancedchemical vapor deposition ("PECVD"). The TEOS decomposes to form anadherent layer 15 of SiO₂. Although the capacitor dielectric isdescribed herein as being silicon dioxide, it should be understood thatother materials can also function as capacitor dielectrics. For example,the capacitor dielectric 15 can be formed from silicon nitride orferroelectric material such as BaTiO₃. Following this, the capacitor isthen masked using a photoresist 16a, which is used to delineate theedges of the capacitor 10. The structure at this point is as diagrammedin FIG. 3.

Referring to FIG. 4, capacitor dielectric layer 15 is etched away with,for example, when the dielectric is silicon dioxide, a wet etch such asethylene glycol/buffered hydrogen fluoride solution, or a dry etch suchas reactive sputter etching. The TiN is resistant to such etches andfunctions as an etch stop. The structure at this point is as diagrammedin FIG. 4.

In the next step the photo resist 16a is stripped off and a layer 17 ofaluminum ("Al") is deposited. The capacitor 10 is formed in the areaswith the remaining capacitor dielectric in which the capacitor layersare titanium-titanium nitride-silicon dioxide-aluminum.

Next, the aluminum layer 17 is patterned with masking material, i.e.photoresist portions 16b and 16c, which are exposed and developed inaccordance with conventional photolithography techniques to define gaps16d for exposing the surface of the integrated circuit to an etchant.The photoresist portion 16b, which defines the limits of the capacitor10, should terminate on the oxide 15. The resulting structure is asdiagrammed in FIG. 5.

Finally, the aluminum is over-etched sufficiently to remove dielectric15 and the titanium-titanium nitride underlayer 14 positioned at thegaps 16d. Then the photoresist portions 16b and 16c are stripped. Theresulting structure is as shown in FIG. 6. Structure 19 is a contact viacontaining conductive layers of aluminum 17 Ti/TiN 14 and provideselectrical access to bottom plate 14 of the capacitor 10 through thepolysilicon layer 12.

Although the subject invention has been described with respect topreferred embodiments, it will be readily apparent to those havingordinary skill in the art to which it appertains that changes andmodifications may be made thereto without departing from the spirit orscope of the subject invention as defined by the appended claims.

What is claimed is:
 1. An integrated circuit comprising:a) a conductivesubstrate; b) a layer of field dielectric formed on said conductivesubstrate; c) a layer of conductive polycrystalline silicon formed onsaid layer of field dielectric; d) first and second laterally spacedapart layers of conductive material formed on said layer ofpolycrystalline silicon and in electrically conductive relationtherewith, each of said spaced apart first and second layers comprisinga layer of titanium and a layer of titanium nitride; e) a layer ofcapacitor dielectric disposed on said first layer of conductive materialin contacting relation with said layer of titanium nitride; f) a thirdlayer of metal disposed on said layer of capacitor dielectric; g) afourth layer of metal disposed on said second layer of conductivematerial.
 2. The integrated circuit of claim 1 wherein said third andfourth layers of metal comprise aluminum.
 3. The integrated circuit ofclaim 1 wherein said first and second layers of conductive material areseparated by an interlevel dielectric.
 4. The integrated circuit ofclaim 1 wherein said capacitor dielectric comprises silicon dioxide.